Trunked communication systems afford a highly efficient use of limited frequency spectrum. In both the private and public markets, more and more businesses, associations, and agencies are specifying trunked systems for their communication needs. In a trunked communication system, a central controller allocates a limited number of channels among many users. Generally, within a trunked system, the many users are organized into fleets, sub-fleets or groups, and individuals. Accordingly, each user has a fleet, group, and an individual identification (ID) code. Since the number of channels is far less than the number of users, the users must request and be granted a channel before they may communicate.
To initiate a group call, it is known for a requesting user to transmit an information packet, commonly referred to as a call request. The call request consists of an ID field containing the requesting user's fleet, group, and individual IDs, and a code which represents the type of call desired (a group call in this example). Generally, call requests are encoded with error correcting and/or other codes to form Inbound Signalling Words (ISWs) to facilitate communication. The central controller receives the ISW and extracts the call request. If a channel is available, the central (controller) formats an information packet, commonly referred to as a channel grant, which comprises the requesting user's fleet, group, and individual IDs, and a channel assignment field, which contains a code representing the channel upon which communications may commence. The central encodes the channel grant into an Outbound Signalling Word (OSW), which is transmitted to all users monitoring a selected one of the available channels designated as the control channel. If there is no channel available, the central formats and returns a Busy OSW. Upon receipt of the channel grant, the requesting user moves to the assigned channel and begins transmitting, while all other users in the same group as the requesting user move to the communications channel as listening units.
To maintain system integrity, the central occasionally transmits system status OSWs which inform the users within the system of various system information and conditions. A system OSW may contain connect tone data, availability of data channels, any limitations or subsequent return of telephone service, or other related information. Reprogramming communication units using a system status OSW is significantly faster than bringing each unit out of the field and in for servicing, which could easily take weeks or months in systems with a thousand or more units, consequently causing extensive loss of communication time.
Quite often, the RF coverage area of adjacent systems overlap. Consequently, a user may receive system status OSWs from two separate systems. This can have negative side effects, especially if the system status OSW contains program data for the communication unit. For example, the system status OSW may contain connect tone data, a sub-audible frequency for inbound signal identification by the communication unit on the assigned communications channel. Because the system may need to change the connect tone frequency at times, such as to avoid interference with a nearby system using the same connect tone frequency, a system status OSW is used to reprogram all the units in the system. If a communication unit receives a system status OSW containing connect tone frequency change data from a neighboring system, the unit may accept the new connect tone, program the new tone, and find that communication in its own system is no longer possible without reprogramming the communication unit manually. Similarly, an OSW incorrectly decoded due to poor signal conditions requires manual reprogramming as well.
Accordingly, a method of using system status OSWs to program communication units with limited influence to decoding nearby system's OSWs and falsing due to poor signal conditions is required.